If you said "blood vessels" or "bronchi" you'd be wrong, but those would both be very good guesses. There's a reason this false-colour image of the Mississippi Delta — photographed by Japan's Advanced Land Observing Satellite — bears such a striking resemblance to ramifying vasculature; as Unpopular Science explains, it all boils down to efficiency:

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To highlight the edges of the branching pattern of river channels, vegetation in this image have been coloured red. While engineers have done their best to control the course of the Mississippi River with a series of levees and artificial channels, it is still difficult to control the 17000 cubic meters of water that flow out of the mouth of the river every second. On its 2320 mile journey from its source, the river has picked up a large amount of sediment. As the water slows as it enters the Gulf, this sediment can no longer be carried by the moving water, and so is deposited.

It's not a coincidence the image shares a striking similarity with blood vessels – a structure hundreds of thousands of times smaller. The fractal pattern that occurs in both is an efficient way to cover a large surface area. It also occurs in the neurons in our brain and in the system of airways in our lungs.